A challenge facing the steel industry in its never-ending quest for greater efficiency is the development of a continuous steelmaking process. While the upstream iron-making and the downstream casting of steel are well-established continuous processes, steelmaking remains essentially a batch operation.
Steelmaking is typically conducted in a circular electric arc furnace. Reduced iron in the form of small spheres or briquets, and optionally some steel scrap, is charged into the furnace and melted by very high power electrodes, creating a power density in the furnace hearth of about 2000 to 2500 kW/m2. The molten metal is protected and partly refined by a liquid slag layer comprised primarily of metal oxides. After the charge is melted, the molten metal is poured or tapped from the furnace for subsequent alloying and casting.
A number of processes for continuous steelmaking have been proposed in the prior art. However, none has proved to be completely satisfactory.
One example of a continuous steelmaking process is proposed in U.S. Pat. No. 4,119,454 (Rath). According to the Rath patent, the steelmaking step is performed continuously in a stationary round or rectangular electric arc furnace. The furnace electrodes are immersed in the slag layer, which acts as a resistive heating element to heat the underlying molten metal layer. In the Rath patent, scrap steel is fed continuously or intermittently into the furnace through one or more openings, and tapping of slag and steel is performed periodically. The metal bath in the furnace is maintained at a volume of about 1 to 10 tap volumes.
The Rath process uses a highly reactive and superheated slag, with the operating temperature of the slag being 40-100° C. above the steel melting temperature and 70-220° C. above the slag melting temperature. Thus, the melting temperature of the slag used in the Rath process is about 30-120° C. less than the steel melting temperature. This superheated slag has very high liquidity and high reactivity, and results in excessive wear on the furnace refractory.
U.S. pat. No. 4,133,468 (Gudenau et al.) describes a method for continuously melting sponge iron (also referred to herein as “direct reduced iron” or “DRI”) in the production of steel having a carbon content of as low as 0.015%. According to Gudenau et al, the furnace electrodes are immersed in a foaming slag layer. The slag is basic, has a CaO/SiO2 ratio sufficient to maintain good liquidity, and contains from 7 to 30% FeO and 5 to 12% MgO. A slag according to this composition may, however, still have high fluidity and high reactivity with respect to the furnace refractory.
U.S. Pat. No. 3,463,269 (Hatch) describes the use of a stationary six-electrode rectangular electric arc furnace in the production of steel. The furnace is charged continuously with sponge iron to produce the final or semi-finished steel. Steel scrap is charged into the furnace at the beginning of the furnace campaign in order to form a pool (“heel”) of molten steel which remains in the furnace below the tap hole at all times. In the Hatch process, the power input is limited by maintaining the metal bath temperature just above the steel melting point to minimize under-cutting of the slag and refractory attack, and to prevent violent boiling due to reaction of carbon with metals in the slag. This method has a number of disadvantages, including the possibility that the furnace will be run with a highly superheated slag of high basicity. Furthermore, the Hatch method does not disclose continuous charging of scrap into the furnace.
Therefore, there is a continued need for a continuous method of steelmaking.